1. Field of the Invention
This invention relates to a sleeveless cylinder block for vehicle engines, which cylinder block does not have a sleeve inserted but has a plated inner cylinder wall, and in particular, to such a sleeveless cylinder block having a plating coating deposited only and securely on the necessary portion, and having an unplated portion at the bottom of the inner wall.
2. Background of the Art
Light weight aluminum alloys have been used in the cylinder heads and cylinder blocks of vehicle engines in order to reduce their weight, but aluminum alloys do not have sufficient resistance to friction abrasion occurring on the inside surface of the cylinders upon which the pistons slide. Thus, cast iron sleeves (liners) are generally installed to make the sliding surface more resistant to abrasion. However, since cast iron sleeve inserts are heavy, efforts to make engines, especially multicylinder engines, more compact and lighter have been focused on the approach of directly increasing the abrasion resistance of the inside surface of the cylinders made of aluminum alloys by plating the inside surface with chrome or nickel, as an alternative to inserting cylinder sleeves. As above, despite the fact that plating needs to be performed on the inside surface of a cylinder, the structures of cylinders for vehicle engines are not designed for plating, and thus plating localized on the inside surface of the cylinder raises complications in many cases. For example, internal combustion cylinders made of aluminum alloys for automobile engines are integrated with cylinder skirts which are further integrated with upper crankshaft bearing portions mounted on and between the cylinders at the bottom of the cylinders, and therefore controlling a plated area is difficult.
Conventionally conducted plating treatments are immersing methods in which a cylinder and an electrode are immersed in a plating liquid which is stored in a tank, and a voltage is impressed therebetween. However, based on the immersing methods, it is difficult to locally plate the cylinder, and parts such as the outside of the cylinder and the inside surface of the cylinder skirt which it is not necessary to plate are also inevitably plated. Further, in immersing methods, the plating speed is impractically slow, and plating productivity is very low. In order to improve productivity of the common plating treatment, a technology to speed up plating processes by impressing a voltage between the inner surface of a cylinder to be plated and an electrode while permitting a plating liquid to flow therebetween has been recently developed. However, even if the flowing liquid plating method is adopted, local plating is not necessarily achieved since it is not easy to control the plating liquid flow as desired. An attempt to plate the inside surface of a cylinder using a flowing liquid plating method concerns the plating liquid flow which takes place only inside the cylinder in which a cover (seal) for sealing plating liquid is placed near the bottom of the cylinder in the cylinder skirt (Japanese Patent Application Laid-open No. 5-222589 (1993). A balloon type seal placed over the bottom of the cylinder in the cylinder skirt can be adapted to seal plating liquid (Japanese Patent Application Laid-open No. 4-333594 (1992)).
However, the above methods involve several problems. That is, when the bottom of a cylinder is not flat, i.e., some portions integrated with the cylinder at the bottom of the cylinder in connection with a crankshaft, it is impossible to fit the seal over the bottom of the cylinder to seal plating liquid at the bottom of the cylinder, and thus a seal must be placed somewhere inside the cylinder skirt, meaning that the portions for a crankshaft will also be plated. Even though there is no obstacle at the bottom of the cylinder so that a seal can placed over the bottom, a plating coating is formed all the way down the bottom of the cylinder and over the brim. The edges of a plating coating are quite fragile in any case, so that when the bottom edge of the cylinder or the portion for a crankshaft undergo mechanical treatment such as mechanical grinding, the plating edge may be cracked, and the cracks may spread into the other pads of the plating coating.
In addition, the above seals placed over the bottom of the cylinder may not be sufficiently secured when used in a high speed plating method in which the speed of plating liquid flow is 1-6 m/sec which improves productivity. If sealing is not complete, the plating liquid will leak through the incomplete pad of the seal, resulting in disturbing the quality control, plating unnecessary pads, and causing hazards in operation. However, since, in the aluminum alloy sleeveless cylinder blocks of the prior art, the cylinder skid, formed below the lower pad of the cylinder, starts approximately at the lower end of the piston, a sealing jig cannot be disposed on any place other than over the bottom of the cylinder in order to plate the inside of the cylinder, and the aforesaid drawbacks cannot be eliminated. In addition, the area between the bottom of the cylinder and the cylinder skirt is relatively weak as compared with the plated area, and thus noise from operation of the engine is readily generated.